Distortion-free automatic volume control amplifier



April 10, 1951 F. w. ROBERTS ET AL 2,543,322

DISTORTION-FREE AUTOMATIC VOLUME CONTROL AMPLIFIER Original Filed Dec. 29, 1944 5%.? O 0 7 E maw wm WWO. m .vm T M am w m F pull amplifiers, for two principal reasons.

Patented Apr. 10, 1951 UNITED STATES PATENT OFFICE DISTORTION-FREE AUTOMATIC VOLUME CONTROL AMPLIFIER Frederick W. Roberts, Fairfield, and Roger 0. Curtis, New Haven, Conn., assignors to Dictaphone Corporation, Bridgeport, Conn., a corporation of New York Original application December 29, 1944, Serial No. 570,280. Divided and this application March 24, 1947, Serial No. 736,752

The present application is a division of patent application Serial No. 570,280, filed December 29, 1944.

In order to produce satisfactory records on the usual recording mediums with normal types H of recorder elements, the level of the signal applied to the recorder element must be maintained within a relatively narrow range regardless of the original range of the sound being recorded. This is especially difificult when a single recording system must be used to record sounds of widely different levels from many sources, such as in the recording of telephone conversations, radio programs or communications, conferences, speeches, and the like. This reduction in the volume range of the sounds being recorded to the limited range required at the recorder is usually accomplished by the provision inan electronic tube amplifier of an automatic volume compression circuit, frequently referred to as an A. V. C. circuit. In the usual operation of such circuits a D. C. voltage is produced which is proportional to the input signal applied to the amplifier if the A. V. C. circuit is of the forward-acting type, or to the output signal of the amplifier if the A. V. C. circuit is of the backward-acting type. This D. C. voltage is applied to the grid of one or more variablegain amplifier tubes, which may be of the super-control or Variable-mu type, to control their amplification factor and therefore the gain of the amplifier in accordance with the strength of the signal.

Ideally such an A. V. C. circuit should operate almost instantaneously to bring the amplifier gain to the proper value before the first portion of any sound or syllable of speech can blast the recorder unit and overcut the recording medium. Experience has shown that in order to obtain a high quality recording the operating time of the A. V. C. circuit, i. e. its compression time, should be about ten milliseconds or less. Such a compression time has been almost impossible to realize practically in single-ended amplifiers, i. e. other than "push- The first reason is that a fast acting A. V. C. circuit having a short compression time causes the magnitude of the average level of the signal passing through the amplifier to vary at 2 such a high rate as to produce audible tone or bump, as it is frequently called. For example,

with a ten milli-second compression time,-the amplifier gain varies at a rate comparable to that of a sound having a frequency of one hunidred cycles persecond. This sound or bump occurs each time the A. V. C. circuit operates and is superimposed upon and interferes with the signal being amplified. In the past, in order to avoid this undesirable effect, it was necessary to increase the compression time of the A. V. C. circuit until its rate of operation was reduced to some value comparable to a subaudible frequency. However, such a remedy is limited by the amount of blasting and overcutting, caused by the reduction in the speed of operation of the A. V. C. circuit, which can be tolerated in the recording system. The second reason is that it is impossible to filter the A. V. C. control voltage satisfactorily in an A. V. C. circuit having a short compression time because such a short action period requires circuits with small time-constants which means circuits having small resistance and capacity values and therefore limited filtering effectiveness. In order to operate satisfactorily the A. V. C. control Voltage should be substantially pure D. C. With insufiicient filtering in the A. V. C. circuit such as results from short time-constant circuits, the A. V. C. potential will be a pulsating D. C. containing an A. C. component corresponding to the signal being amplified, and this component will be impressed on the grids of the amplifier .tubes along with the signal, thus causing considerable distortion inthe amplified signal. Normally, this effect can be overcome only by increasing the compression time of the A. V. C. circuit because an A. V. C. circuit which operates at a rate comparable to some audible frequency must of necessity transmit that frequency and therefore cannot be designed to eliminate this distortion voltage by filtering. Thus, single-ended amplifiers having a sufiiciently rapid A. V. C. action for high quality recording are subject to the disadvantages of bumping and distortion. Further, rapid acting A. V. C. circuits may cause blocking of the amplifier tubes due to large swings in the A. V. C. potential. Although these disadvantages may be partially overcome by the use of a push-pull transformer-coupled amplifier in which the A. V. C. potential is applied in phase to the grids of the first two tubes, even such an amplifier is subject to blocking and distortion when used quality transformers and other expensive and complex circuit elements.

Accordingly, therefore, it is an object of this invention to provide an electronic tube amplifier having a rapid action A. V. C. circuit which does not cause bumping, distortion, or blocking, It is a further object to provide such an amplifier which is particularly adapted for use in sound recording and reproducing apparatus for recording sounds from sources of widely divergent levels, and to provide novel circuits for controlling its operation.

These and other apparent objects and advantages of this invention are obtained by the means described in the following specification and may be more readily understood by reference to the accompanying drawing which shows schematically an electric recording and reproducing apparatus embodying the invention.

Referring to the drawing, the recording and reproducing apparatus shown therein generally comprises a source of signal to be recorded, gen erally indicated by the blocked area ltd, such as a microphone, telephone, radio, or the like; a pro-amplifier stage, generally indicated at 152, including a voltage amplifier tube V-i; an amplifier A. V. C. stage, generally indicated at I94, including A. V. C. controlled amplifier tubes V-2 and V3; an A. V. C. potential amplifying and rectifying circuit, generally indicated at Hit, including an amplifier tube V i and a rectifier i tube V5; a phase-inverter cathode-follower circuit, generally indicated at H38, including vacuum tubes V% and Vl'; a power amplifying stage,

generally indicated at HG, including a power amplifier tube V-S; a power supply for all of these circuits generally indicated at H2, including a rectifier tube V9; a recorder and reproducer mechanism, generally indicated at H5; and an output circuit, generally indicated at ll l, which may be selectively connected either to an electrical recorder element l3!) or a loudspeaker unit H8 dependent upon whether the apparatus is conditioned for recording or reproducing.

The signal to be record from the signal source IE8 is impressed through an input transformer ll-I to the grid l2 of the pro-amplifier triode V-l of the pie-amplifier stage I62. The preamplifier tube V-i is provided with the customary cathode bias resistor R4 with its bypass condenser C-IB, and a plate load resistor R5 connected to a suitable positive high voltage terminal i2ii in the power supply i $2. The amplified signal appearing across the plate resistor R.5 of tube V-l is. applied through a coupling condenser C--l to the control grid 22 of the remotecut-off screen-grid pentode V-'-2 in the amplifier A; V. C. stage 29 3 where it is amplified in'a normal manner so that the amplified signal appears across its plate load resistor R-il which is connected to a positive high voltage terminal [24 in power supply H2. The suppressor grid 25 of the ,pentode V-2 is connected to the cathode 2| in the usual manner, and V--2 is provided with the customary screen-grid resistor R-9 and bypass condenser C-8. The control grid 22 of tube V-2 is arranged to be grounded through a relay contact K8 to prevent the appearance of any parasitic signals on the grid 22 when the apparatus is conditioned for reproducing, as will be described hereinafter.

The amplified signal from pre-amplifier stage W2 is also fed to the A. V. C. circuit I06 through a condenser C5 to the control grid 42 of a remote-cut-oif screen-grid pentode V& which amplifies the A. V. C. potential before it is rectified by the A. V. C. rectifier V--5. This A. V. C. potential amplifying tube V- l is provided with the usualgrid return resistor R 28, cathode bias resistor R-2l with its bypass condenser Cl'!, screen-grid resistor R25 with its bypass condenser Cl9, and plate load resistor fir-24 connected to a positive high voltage tap |2fl in the power supply H2. Its suppressor grid 45 is connected to its cathode ll in the usual manner. The amplified A. V. C. potential which appears across the plate resistor R-24 of amplifier tube v-as is applied through a. coupling condenser C-! 5 to the A. V. C. rectifier V5. This rectifier V5 is a double diode connected in a voltagedoubler arrangement as shown in the diagram so that the rectified A. V. C. pulsating D. C. potential appears across resistors R122 and Rr-23, shuntedby a condenser Ci l. Briefly, this voltage-doubler rectifier arrangement operates as follows: during the portion of the cycle of the signal impressed on grid 52 of A. V. C. potential amplifying tube V-4 when the grid is going negative, the current through this tube decreases, thus decreasing the voltage drop across plate resister R--24 and increasing the positive potential above ground of plate as and the side of coupling condenser Cl5 connected thereto. This causes condenser Ci5 to be charged to a. voltage equal to the peak value of this voltage change through cathode 53 and anode 54 of one of the diodes of rectifier tube V5. During the next half cycle of the signal voltage when the grid 42 of tube V l is goin positive, the current through tube V-d increases, thus increasing the voltage drop across plate resistor R-24 and thereby decreasing the potential above ground of the plate of condenser Cl5 connected to this resistor and the plate 33 of tube V4. This, in turn, causes condenser C--l5 to discharge through cathode 5| and anode 52 of the other diode of rectifier tube V-5 and condenser C--M thereby causing condenser C-i i to be charged to a voltage substantially equal to the voltage change across plate resistor lit-24 of tube V4 during one complete cycle of the signal voltage. This discharge of condenser 0-15 prepares it for the next negative half cycle of the signal voltage and at the same time produces across resistors lit-22 and 3-23 the D. C. potential to be used as the A. V. C. control potential and which is proportional to the magnitude of the signal potential. Part of this potential is applied as a. negative bias to the control-grid 42 of the A. V. C. potential amplifying pentcde V4 by connecting the juncture between resistors R22 and 18-23 through the grid return resistor RF-ZB to the control .grid 42 of tube V4. With this novel arrangementoi applying a portion of the A. V. C. D. C. potential back to the grid of the A. V. C. potential-amplifying tube V4, an A. V. C. effect is superimposed upon the original A. V. C. effect so as to greatly increase the signal range over which the A. V. C. circuit is effective. By choice of ten milli-seconds.

of suitable values for resistors R--22 and Rr23 determining the proportion of the A. V. C. D. C. potential fed back to tube V--4l, the slope of the A. V. C. operating characteristic curve can be predetermined as desired.

The A. V. C. D. C. potential developed across resistors Rr22 and R-23 is applied through a connection from the negative end of resistor R-22 through a grid resistance R.5 to the control grid 22 of the amplifier A. V. C. tube V2 above-described and, in the same phase, through a similar resistance R--8 and a relay contact K-l hereinafter to be described, to the control grid 32 of the other remote-cut-ofi pentode tube V3 of the amplifier A. V. C. stage IM. Tube V-3 is identical to tube V2 and is similarly provided with the usual plate load resistor R-IZ connected to high voltage terminal I24, and screen grid resistor Rl andbypass condenser C-1. A series circuit comprising a resistor R1 and a condenser C- is connected from the control grid 32 of tube V-3 to ground so thatthe impedance across the input of tube V3 is substantially equal to the impedance across the input of tube V-Z due to the plate circuit impedance of Vl and the grid circuit impedance of V4, so that the response of tubes V-2 and V3 to changes in the A. V. C. potential applied thereto through resistors R-5 and Rr-B, respectively, is substantially identical. The control grid 32 of tube V3 may also be connected through a relay contact K2, alternate to the above-mentioned contact Kl, to a grid return resistor R30 and, through a filter formed of a resistor R.26 and a shunting condenser C-3, to the reproducing element I32 of the recorderreproducer mechanism 1 l6 when the apparatus is conditioned for reproducing and tube V3 is operated as the input amplifier tube. At the same time the A. V. C. potential is ground by another relay contact Kl connected to resistor R8. The A. V. C. amplifier tube V4, the components of its associated circuits, and the A. V. C. rectifier tube V-5 are all suitably selected sothat condenser C-l4 can be charged rapidly to the full A. V. C. potential, i. e. in a period of the order Inasmuch as the charging time of condenser Cl4 determines the compression period of the A. V. C. circuit this permits the A. V. C. response of the amplifier to be made sufiiciently rapid to permit high quality recording without blasting or overcutting.

With the circuit thus far described, it is evident that the voltage which appears across the plate resistor R,II of tube V-2 is a combination of the signal from the source IOU amplified by tubes VI and V2, plus any bump produced by operation of the A. V. C. circuit, plus any distorting voltage which may come from the v A. V. C. circuit due to the presence in the A. V. C.

potential of an A. C. component resulting from some of the signal voltage leaking through the A. V. C. circuit caused by lack of sufiicient filtering in the A. V. C. circuit. On the other hand, the voltage which appears across the plate resistor R-I2 of tube V3 is merely a combination of any bump voltage produced by operation of the A. V. C. circuit plus any distortion voltage appearing in the A V. C. potential due to the presence of an unfiltered A. C. component of the signal. These bump and distortion volt ages appearing across resistor R-I2 are in phase with, and of substantially equal magnitude to, the two similar voltage components appearing acrossplate resistor R-Jl of tube V2.

These voltages appearing across resistors R-I I and R--l2 are impressed, respectively, upon the control grids 62 and 12 of the two triodes V--6 and V'! connected in the phase-inversion cathode-follower circuit H38 in such a manner, hereinafter to be described, that the A. V. C. bump and distortion voltages appearing across resistor R,! I are cancelled by the corresponding voltages appearing across resistor Rl2, thereby leaving only the signal voltage to pass on .to the power amplifier stage H0. This is accomplished by the novel circuit connections shown in the drawing. The amplified voltage appearing across resistor R-ll comprising components resulting from the signal voltage, the A. V. C. bump" voltage, and the A. V. C. distortion voltage, is reduced by a voltage divider formed of a resistor R.-l3 and a resistor R-29, and this reduced voltage is appliedthrough a coupling condenser Cl0 to the control grid 62 of one of the triodes V- 6 of the cathode-follower phase-inverter circuit I118. The amplified voltage appearing across the plate resistor R-l2 of tube V3 comprising components resulting from the A. V. C. bump and distortion voltages is fed through an attenuator R-48, connected in parallel with resistor R-I2, and a coupling condenser Cll to the control grid 12 of the other triode V! in the cathode-follower phase-inverter circuit I08. The cathodes SI of tube V-fi and H of tube V-'! are connected together and to ground through a grid biasing resistor Rl8 and a cathodefollower load resistor R-l1. The grid returns of the tubes ,V6 and V-! are made through grid resistors R--I5 and R-Ifi, respectively, connected together and to the junction of resistors Rll and-Rl8. Resistance R,l8 is of such value as to provide a normal operating grid bias for the tubes -V6 and V--'!, whereas cathode follower resistor R-l'l is a'much higher resistance than Rl 8 and forms part of the plate load of these tubes, as is usual'in normal cathodefollower operation. The remainder of the plate load of these tubes is provided by two resistors R-IB and R2B connected from the anodes 63 and T3 of tubes V6 and Vl, respectively, to a suitable high voltage terminal I22 in the power supply H2. The operational characteristics of the cathode-follower phase-inverter circuit just described are such that any voltage impressed on the control grid of one of the tubes, e. g. tube V6, not only appears across the plate resistor of that tube, e. g. plate resistor Rl9, but also appears across the plate resistor of the other tube, e. g. plate resistor R-ZB of tube V--'l, in' approximately equal magnitude but in opposite phase. Without going into a rigorous analysis of the operational characteristics of this circuit which bring about these results, the cause of these efiects may be understood from the following brief explanation. When the control grid 62 of tube V6 becomes more positive its increased cathode-anode current causes the voltage drop across cathode-follower resistor R-l'l to increase, thus causing the cathode 6| likewise to become more positive with respect toground, thus following the potential applied to the control grid 62 while the voltage drop across plate resistor R-l9 increases simultaneously. At the same time, assuming that no signal is impressed on the control grid 12 of tube V-l so that its grid remains at a fixed potential with respect to ground, the cathode H of tube V-I, being connected to the cathode 6! of tube V6, becomes more positive with respect to its control grid 12,

i. e. its control grid becomes more negative with respect to its cathode, thus reducing its cathodeanode current and hence reducing the voltage drop across its plate resistor R-2ll at the same time that the voltage drop across the plate resistor R--i 9 of tube V@ is increased. Thus any voltage impressed on the control grid of one of the two tubes V- S and V-'I will appear across the plate resistor of that tube and also across the plate resistor of the other tube but in' reverse phase.

The novel and advantageous results achieved by the circuit just described are now apparent. The voltage appearing across plate resistor R--! l of tube V-Z, comprising the signal voltage plus the A. V. C. bump voltage plus the A. V. C. distortion voltage, is impressed on the control grid 62 of tube V-B so that this voltage appears across the plate resistor R-I9 of tube V-% and at the same time it appears in inverse phase cross the plate resistor R-Zi! of tube V-'I. Simultaneously, the voltage appearing across plate resistor R.-i2 of tube V3, comprising the A. V. C. bump voltage plus the A. V. C. distortion voltage, is impressed upon'the control grid i 2 of tube V-i so that it appears across the plate resistor R-ZG of tube V-'I in phase with the corresponding voltage appearing across R-I9 as a result of the bump and distortion voltages impressed on grid 62. operation of this circuit, it also appears in inverse phase across resistor R-IB. Thus, the total voltage appearing across the plate resistor R- lS of tube V-E is equal to the sum of the signal voltage plus the A. V. C. bump voltage plus the A. V. C. distortion voltage, due to the voltage applied to the grid 62 of tube V-5, plus the two last-named voltages in anegative sense, their phase being inverted, due to the voltage applied to grid 12 of tube V-l, so that the net voltage appearing across resistor R-IB is the signal voltage alone, the A. V. C. bump and distortion voltages being cancelled out.

An actual operation, it is an inherent charac teristic of the cathode-follower phase-inverter circuit above-described that when a Voltage is impressed on the grid of one tube the voltage appearing across the output resistor of that tube is slightly larger than the inverse phase voltage appearing across the output resistor of the other tube. One'reason for this effect is that the effective cathode to grid voltage applied to the first tube is slightly greater than that applied to the second tube because of the feedback action of the common cathode-follower resistor R! 1. Because of this circuit characteristic just described, in order to achieve complete cancellation of the A. V. C. bump and distortion voltages, the abovedescribed voltage-divider circuits, formed of the two resistors R-IB and R2'9, and of the attenuator Ri8, are provided in parallel with the plate resistors Pt! 1 and R-IZ, respectively, of tubes V-2 and V3. This permits the ratio of the input voltages to tubes V-S and V--1 to be set by a suitable adjustment of the attenuator Pa e so that the A. V. C. bump plus distortion voltage components impressed on the control grid I2 of tube V-'I from plate resistor R-l2 of tube V-3 are slightly greater than the corresponding voltage components impressed on control grid '52 of the tube Vii from the plate resistor lit-I i of tube V-2, as above described.

The signal voltage, with the A. V. C. bump and distortion voltage components cancelled out, appearing across plate resistor R-I9 of tube V-B However, due to the novel i impressed on the power amplifier stage I I0 through a coupling condenser 0-42, a recording volume control potentiometer R3'I, and a relay contact K3, hereinafter to be described, on the control grid 82 of a beam-power type power amplifier tube V--8.

When the apparatus is conditioned for reproducing, the signal is fed from reproducer element i312 through tube V3 to the grid of tube V-'I where it appears across the plate resistors R-I9 and R28 of tubes V6 and V-'I. In order to permit the volume of reproduction to be set at a different value from the recording volume, a separate reproducing volume control is provided by utilizing the signal appearin across plate resistor 'R-20 instead of that appearing across R-i9. This is accomplished by impressing this signal through a coupling condenser C-I8, a reproducin volume control potentiometer R-36, and a relay contact K-4, alternate to contact K-3, on to the grid 82 of power amplifier tube V-8.

The circuit of the power amplifier stage I!!! is of conventional design. The anode 83 of tube V-8 is connected to one side of the primary of an output transformer T-2, the other side of which is connected to a suitable high voltage tap !2ll in the power supply II2. Its screen grid 84 is connected to another tap I22 therein of slightly lower voltage. The customary cathode biasing resistor P,2I, with its bypass condenser CI3, is connected to the cathode BI, and, in order to provide a suitable inverse feed-back in this power amplifier circuit, the cathode lead from resistor R2I is connected to one side of the secondary of output transformer T2, the other side of the secondary being connected to the ground circuit. The high side of the secondary of the output transformer T-2 also is connected through a relay contact K5 to the recorder element I30, the other side of which is connected to the grounded side of the secondary of transformer T---2. By means of a relay contact K--6, alternate to contact K5, the output of the power amplifier stage IIG from the secondary of transformer T-2 can be connected to the loudspeaker The recorder element I30 and reproducer element I32; which may be of any suitable type such as those shown in U. S. Patents No. 2,181,437 to Norton or No. 2,318,828 to Yerkovich, of the recorder-reproducer mechanism I I6 are mounted on a mechanical control mechanism, generally indicated at I34, operated by control lever I36,

, whereby either the recorder element or the reproducer element may be operatively positioned with respect to asuitable recording medium such as the wax cylinder I38. The control lever I38 is adjustable to three positions, a recording position in which the recorder I36 is moved into operative position with respect to the record B8, 2. reproducing position in which the reproducer element I32 is moved into operative position with respect to record 538, and a neutral position in which neither the recorder nor reproducer elements are in operative relationship to the record. Four contact points 49, I42, I44 and I48 are associated with the mechanical control mechanism I34so that contact is made between points I48 and 542 when the control lever I38 is in recording position, and between contacts I44 and I46 when the control lever I36 is in reproducing position, and in which the circuits between contacts I48 and I42 and between I44 and I46 are open when the control lever 136 is in neutral tubes.

position. The function of these contacts will be hereinafter described.

The power supply H2 is of the conventional full-wave rectifier type comprising a power supply transformer T-3 connected to an alternating current supply line I50 through a fuse F-I and a double-pole single-throw off-on switch S-I, a full-wave rectifier tube V--9 having a cathode SH and two anodes 92 connected in the usual manne to the transformer T-3 and to a filter.

system comprising a plurality of filtering resistors R-42, R-43, R--44, R45, a bleeder resistor R46, and filter condensers C2I, C23, C25, C 26, and C--28 connected in the usual manner, providing D. C. operating potentials be-.

tween 2. negative terminal I28 and a plurality of positive high voltage terminals I20, I22, I24 and I26 connected to the various circuits as abovedescribed.

This power supply circuit is of the usualtype K-I, K--3, and K5 when it is deenergized,

and to close contacts K2, K4, K-6, K- I and K-B when it is energized. The filter resistor R42 has a resistance approximately equal to the resistance of relay coil MI. With this arrangement, when the off-on switch S-I is closed and the apparatus is in operative condition, the plate currents of all of the tubes pass through resistor R42 and relay coil MI. This relay coil MI and resistor R42 are connected to the above-mentioned control contacts I40, I42, I44 and I45 associated with the recorder-reproducer mechanical control mechanism I34. Terminal H9 of resistor R'42 is connected to contact point I46, terminal I20 of relay coil MI is connected to contact I40, and a terminal I2I between relay coil MI and resistor R42 is connected to contact points I42 and I44. Thus,

when recording-reproducing lever I36 is in recording position, thus closing the circuit between contacts I40 and I42, relay coil MI is short circuited and therefore deenergized; whereas, when control lever I36 is in reproducing position, thus closing the circuit between contacts I44 and I46, resistor R-42 is short circuited, thus increasing the current through relay coil MI so that it is fully energized. When recording-reproducing control lever I36 is in neutral position both relay coil MI and resistor Rr42 are in series with the plate supply of all the This introduces a higher than normal resistance in this circuit and thus decreases the plate current to all the tubes. This arrangement tends to decrease the load on the tubes when the sistor R.-8 and contact KI to the grid 32 of tube V--3.

It also connects the recorder volume control potentiometer R-31 through contact K--3 to the grid 82 of the power amplifier tube 10 V8.v Finally, it connects the high side of the secondary of the output transformer T 2 through contact K5 to the recorder element I30. Simultaneously the recorder element I30 is positioned in operative relationship to the rec- 0rd I38. When a signal from the signal source I00 is introduced to the input of the apparatus this signal'is amplified by tubes V-I, VZ, and

V8 and applied to the recorder'element I30. At the same time a portion of the signal is amplified and rectified by circuit I06 into an A. V'. C. control potential which controls the amount'of amplification of the signal by tube V2.in.the amplifier A. V. C. stage I04. Through the action of tube V-3 and the cathode-follower phaseinverter stage I08, formed of tubes V-6 and V-i, any bump or distortion voltage caused by the operation of the A. V. C. circuit" is cancelled cut in the output of tube V-6. in the manner above-described, so that the signal amplified by tube V-8 and applied to recorder element I30 corresponds to the original signal undistorted by the operation of the A. V. C. circuit and by any A. V. C. bump. The average volume of the signal being recorded is determined by the setting of the recording volume control R3'I. When the recording-reproducing control lever I36 is placed in reproducing position, relay coil MI is energized, thereby opening contacts KI, K3 and K5 and closing contacts K-2, K-4, K'6, K'I and K-8; This grounds the control grid 22 of amplifier tube V2 through contact K8 to obviate the possibility of a signal or other interfering voltage being impressed upon' the grid of this tube during reproduction. The A. V. C. control'voltage from the output of rectifier V5 is rendered inoperative by being disconnected from the control grid32 of tube V-3 by opening of the contact KI and by being grounded through the closing of contact K'I. At the same time the reproducing element I32 is connected through contact K-2 to the control grid 32 of tube V3, whence it is amplified and appears across the plate load resistor R20 of tube VI from which it is fed through the reproducing volume control R.-36 and contact K-4 to the control grid 82 of power amplifier tube V8. It is further amplified by tube V8 and passed through the output transformer T--2 and contact K 6 to the reproducing loudspeaker H8. As above mentioned, the volume during reproduction is determined by the setting of reproducing volume control R35.

As many embodiments may be made in the.

above invention and as many changes may be made in the embodiment above described, it is to be understood that all matters hereinbefore set forth or shown in the accompanying drawing is to be interpreted as illustrative only and not in a limiting sense.

We claim:

1. For use in a sound recording electronic tube 11 connections and connected in a cathode-follower phase-inverted circuit, means connecting the output circuit of one of said amplifier stages to the input connection of said first tube, means connecting the output circuit of the other of said amplifier stages to the input connection of said second tube, and means connecting the output connection of said first tubeto the output circuit of said amplifier system, where-by Signal distortion caused by operation of the A. V. C. circuit is substantially eliminated in the output of said amplifier system.

2. For use in a sound recording electronic tube amplifier system having an input and an output circuit, an amplifier A. V. C. circuit, comprising, in combination, circuit means producing a D. .C. A. V. C. potential which is a function of the signalbeing amplified, two amplifier stages each including a remote cut-off pentode tube having an anode, a cathode, and a control grid, means applying said A. V. C. potential as a variable grid bias to bothof said pentode tubes, means applying the signal to be amplified to the grid circuit of one of said pentode tubes, two triode vacuum tubes each having an anode, a cathode, and a control grid and connected in a cathode-follower phase inverter circuit, said last named circuit including first and second cathode resistors connected to the cathodes of said tubes and first and second grid resistors connected respectively between the control grids of said tubes and the junction of said cathode resistors means connecting the plate circuit of one of said pentode tubes to the grid circuit of one of said triode tubes, means connecting the plate circuit of the other of said pentode tubes to the grid circuit of the other of said triode tubes, and means supplying an A. V. C. controlled signal to theoutput circuit of said amplifier system from the plate circuit of one of said triode tubes, whereby signal distortion caused by operation of the A. V. C. circuit is substantially eliminated in said A. V. C. controlled signal supplied to said amplifier system.

3. For use in a sound recording electronic tube amplifier system having an input and an output circuit, an amplifier A. V. C. circuit, comprising, in combination, circuit means producing an A. V. C. potential which is a function of the signal being amplified, two variable-gain amplifier stages having input and output circuits, means applying said A. V. C. potential as a variable operating bias to both of said amplifier stages, means applying the signal to be amplified to the input circuit of one ofsaid amplifier stages, two vacuum tubes each having input and output connections and connected in a cathode-follower phase-inverter circuit, means connecting the output circuit of one of said amplifier stages to the input connection of one of said last-named tubes, means connecting the output circuit of the other of said amplifier stages to the input connection of the other of said last-named tubes, separate voltage control means for adjusting the ratio of the input voltages applied to said two last-named tubes, and means utilizing the output signal from one of said last-named tubes as the output signal applied to the output circuit of said amplifier system, whereby signal distortion caused by operation of the A. V. C. circuit is substantially eliminated in the output of said amplifier system.

4. In an electronic tube amplifier system havingan input and an output circuit for use in sound recording equipment, an A. V. C. amplifier portion comprising, in combination, means for providing a D. C. A. V. C. potential which is a function of the signal being amplified, a first amplifier stage including a first variable-gain vacuum tube, a second amplifier stage including a second variable-gain vacuum tube, said first and second tubes each having an anode, a cathode, and a control grid, the cathode of said second tube being connected to that of said first tube and to a common ground circuit, means for applying the signal being amplified to said first amplifier stage between the control grid of said first tube and said common ground circuit, means for applying said A. V. C. potential to the controi grids of both of said tubes to supply a volumecontrolled bias thereto, third and fourth vacuum tubes each having a cathode, anode and control grid, means for coupling the plate of said first tube to the control grid of said third tube, resistance-capacitance means for coupling the plate of said second tube to the control grid of said fourth tube, circuit means connecting the cathodes of said third and fourth tubes together and to said ground circuit through a common impedance forming part of the load circuit of said tubes, biasing means for supplying a bias potential between the cathodes and the control grids of said third and fourth tubes, two plate impedances each connected to the anode of one of said third and fourth tubes and each forming the remaining portion of the anode load of its respective tube, conductive means connecting said two last-named plate load impedances together and through a source of positive anode potential to said ground circuit, means for supplying the signal to the output circuit of said amplifier system from said A. V. C. amplifier portion from between the anode of either of said third or fourth tubes and the ground circuit, whereby any distortion of the amplified signal due to operation of the A. V. C. circuit is substantially eliminated in the output circuit of said amplifier portion.

5. In an electronic tube amplifier system having an input and an output circuit for use in sound recording equipment, an A. V. C. amplifier portion comprising, in combination, means for providing a D. C. A. V. C. potential which is a function of the signal being amplified, a first amplifier stage including a first variable-mu vacuum tube, a second amplifier stage including a second variable-mu vacuum tube, said first and second tubes each having an anode, a cathode, and a control grid, the cathode of said second tube being connected to that of said first tube and to a common ground circuit, means for applying the signal being amplified to said first amplifier stage between the control grid of said first tube and said common ground circuit, circuit means for applying said A. V. C. potential to the control grids of both of said tubes to supply a variable operating bias potential thereto, third and fourth vacuum tubes each having a cathode, anode, and control grid, resistance-capacitance means for coupling the plate of said first tube to the control grid of said third tube, resistancecapacitance means for coupling the plate of said second tube to the control grid of said fourth tube, voltage control means associated with coupling means for adjusting the ratio of the input voltages applied to the grids of said third and fourth tubes, means connecting the cathodes of said third and fourth tubes together and to said ground circuit through a common impedance forming part of the anode load of said tubes, biasing means for supplying a bias potential between the cathodes and the control grids of said third and fourth tubes, two plate load impedances each connected to the anode of one of said third and fourth tubes and each forming the remaining portion of the anode load of its respective tube, means connecting said two last-named plate load impedances together and through a source of positive anode potential to said ground circuit,

means for supplying the signal to the output circuit of said amplifier system from said A. V. C. amplifier portion from between the anode of either said third or fourth tubes and the ground circuit, whereby any distortion of the amplified signal due to operation of the A. V. C. circuit is substantially eliminated in the output circuit of said amplifier portion.

6. In an electronic tube amplifier for use in sound recording equipment, an A. V. C. amplifier portion comprising, in combination, means for providing a D. C. A. V. C. potential proportional to the signal being amplified, a first amplifier stage including a first variable-mu vacuum tube, a second amplifier stage including a second variable-mu vacuum tube, said first and second tubes each having an anode, a cathode, and a control grid, the cathode of said second tube being connected to that of said first tube and to a common ground circuit in said amplifier portion, circuit means for applying the signal being amplified to said first amplifier stage between the control grid of said first tube and said common ground circuit, coupling means for applying said A. V. C. potential to the control grids of both of said tubes through individual grid resistors to supply a 35 volume-controlled bias thereto, third and fourth vacuum tubes each having a cathode, anode, and

control grid, capacitive means for coupling the plate of said first tube to the control grid of said third tube, capacitive means for coupling the plate of said second tube to the control grid of said fourth tube, means connecting the cathodes of said third and fourth tubes together and to said ground circuit through a first common resistor acting as a self -biasing means for said tubes and a second common resistor forming part of the anode load of said tubes, connection means for applying the bias potential from said first common resistor to the control grids of said third and fourth tubes through individual grid resisters, two plate resistors each connected to the anode of one of said third and fourth tubes and each forming the remaining portion of the anode load of its respective tube, means connecting said two last-named plate load resistors together and through a source of positive anode potential to said ground circuit, means for supplying the signal to the output circuit of said amplifier; system from said A. V. C. amplifier portion across one of said last-named plate load resistors, whereby any distortion of the amplified signal due to operation of the A. V. C. circuit is substantially cancelled out in said plate load resistor.

FREDERICK W. ROBERTS. ROGER C. CURTIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,092,496 Branson Sept. 7, 1937 2,179,414 Konkle Nov. 7, 1939 

